Analytical tool

ABSTRACT

The present invention relates to an analytical tool ( 1 ) comprising a substrate ( 10 ), a capillary ( 13 ) which is formed on the substrate( 10 ) and into which a sample liquid is to be loaded by movement of the sample liquid in the capillary. The substrate ( 10 ) is provided with a liquid movement preventer for preventing the sample liquid loaded into the capillary ( 13 ) from moving further. Preferably, the liquid movement preventer includes a stepped portion ( 18 B) projecting from the substrate or a recess provided at the substrate.

TECHNICAL FIELD

The present invention relates to an analytical tool used for analyzing aparticular component contained in a sample liquid.

BACKGROUND ART

As an easy method for measuring a glucose level in blood, a disposablebiosensor is used (See JP-B2-8-10208, for example). An example of suchbiosensor is shown in FIGS. 11 and 12 of the present application. In theillustrated biosensor 9, a responsive current which is necessary for thecomputation of a blood glucose level is measured by utilizing a workingelectrode 90 and a counter electrode 9. The biosensor 9 includes asubstrate 92, and a cover 94 stacked on the substrate via a spacer 93formed with a slit 93 a. These components 92-94 define a capillary 95 onthe substrate 92. The capillary 95 is utilized for moving blood bycapillary action and retaining the blood. The capillary 95 communicateswith the outside through an introduction port 96 for introducing bloodand an air vent 97 for discharging air from the capillary 95 when theblood moves in the capillary 95.

On the substrate 92 are provided an insulating layer 98 and a reagentportion 99. The insulating layer 98 covers the working electrode 90 andthe counter electrode 91 while exposing opposite ends 90 a, 90 b of theworking electrode 90 and opposite ends 91 a, 91 b of the counterelectrode 91. The reagent portion 99 covers the ends 90 a, 91 a of theworking electrode 90 and the counter electrode 91 and is in a solidstate containing oxidoreductase and an electron mediator.

To measure a blood glucose level, with a biosensor 9 mounted to aconcentration measuring apparatus (not shown), blood BL is introducedinto the capillary 95 through the introduction port 96, as shown in FIG.13. In the capillary 95, the movement of the blood BL stops at the edge97 a of the air vent 97, and the reagent portion 99 is dissolved by theblood BL, whereby a liquid phase reaction system is established. Avoltage can be applied to the liquid phase reaction system by the powersource of the concentration measuring apparatus through the workingelectrode 90 and the counter electrode 91. The responsive current uponthe voltage application can be measured at the blood glucose levelmeasuring apparatus (not shown) by utilizing the working electrode 90and the counter electrode 91. The responsive current is obtained as thereflection of the amount of electrons transferred between the electronmediator and the end 90 a of the working electrode 90 in the liquidphase reaction system. Thus, the responsive current relates with theamount of the electron mediator which exists around the workingelectrode 90 and which is capable of transferring electrons between theend 90 a of the working electrode 90.

However, when the concentration measurement is performed by using thebiosensor 9, the measurement result is sometimes higher than the actualconcentration. To find out the cause, the inventors of the presentinvention measured the change of oxidation current with time by usingsome samples. As a result, it is found that, in some cases, as is in thetime course of oxidation current shown in FIG. 14, the oxidationcurrent, which should decreases monotonically in normal circumstances,suddenly increases instantaneously, as circled in the figure. When sucha phenomenon happens to occur at the time point for measuring theoxidation current for computing the blood glucose level, the computationresult becomes higher than the actual blood glucose level.

The inventors of the present invention checked a plurality of samples inwhich the above-described phenomenon was seen. As a result, it was foundthat, as a feature common to these samples, blood BL had reached beyondthe edge 97 a of the air vent 97 on the surface of the substrate 92, asshown in FIG. 15. On the other hand, in the samples in which the suddenincrease of the oxidation current did not occur, blood BL was stopped atthe edge 97 a of the air vent 97 (See FIG. 13).

Conceivably, from the above difference, the sudden increase of oxidationcurrent is caused by the remove of the blood BL, i.e., the phenomenonthat the blood BL once stopped at the edge 97 a of the air vent 97 movesbeyond the edge 97 a of the air vent 97.

Specifically, when voltage is applied to the liquid phase reactionsystem including the blood BL, electrons are transferred between theelectron mediator and the end 90 a of the working electrode 90.Therefore, in the state in which the movement of the blood BL issuspended, the proportion of reductant is low at the surface of the end90 a of the working electrode 90, so that the oxidation currentdecreases. In this state, when the blood BL moves, reductant moves fromthe introduction port 96 side to the surface of the end 90 a of theworking electrode 90, so that the proportion of the reductant at thesurface of the end 90 a temporarily increases. As a result, the amountof electrons transferred between the reductant and the surface of theend 90 a of the working electrode 90 suddenly increases, so that theoxidation current does not monotonically decrease but increasestemporarily.

DISCLOSURE OF THE INVENTION

An object of the present invention is to prevent, in performing analysisof a sample by using an analytical tool, the sample liquid loaded into acapillary formed on the substrate of the analytical tool from movingfurther so that the sample can be analyzed properly.

According to the present invention, there is provided an analytical toolcomprising a substrate, a capillary which is formed on the substrate andinto which a sample liquid is to be loaded by movement of the sampleliquid in the capillary. The substrate is provided with a liquidmovement preventer for preventing the sample liquid loaded into thecapillary from moving further.

For example, the liquid movement preventer includes a stepped portionprojecting from the substrate. For example, the stepped portioncomprises a conductive layer formed on the substrate and an insulatinglayer covering the conductive layer.

For example, the analytical tool further comprises a plurality ofelectrodes provided on the substrate for applying voltage to the sampleliquid.

For example, the conductive layer is formed as a dummy electrode whichdoes not contribute to the voltage application to the sample liquid. Thedummy electrode is formed simultaneously with the plurality ofelectrodes.

For example, the plurality of electrodes include a detection electrodefor detecting whether or not the sample liquid of an amount necessaryfor analysis is supplied into the capillary. In this case, theconductive layer may be provided by the detection electrode.Alternatively, the conductive layer may be provided by an electrodeother than the detection electrode.

The analytical tool may further comprise an air vent for discharging airfrom the capillary in moving the sample liquid in the capillary. In thiscase, the insulating layer includes an opening which exposes part of theelectrodes and which extends along the capillary. Preferably, as viewedin a thickness direction of the substrate, the most downstream point ofthe opening in a flow direction of the sample liquid is located on thesame line or almost same line as the most upstream point of the air ventin the flow direction of the sample liquid.

The liquid movement preventer may include a recess provided at thesubstrate.

For example, the recess comprises a through-hole penetrating through thesubstrate. Preferably, when the analytical tool includes an air vent,the air vent is arranged coaxially or generally coaxially with thethrough-hole in the thickness direction of the substrate.

Preferably, as viewed in the thickness direction of the substrate, themost upstream point of the recess in a flow direction of the sampleliquid is located on a same line or almost same line as the mostupstream point of the air vent in the flow direction of the sampleliquid.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a biosensor according to afirst embodiment of the present invention.

FIG. 2 is a sectional view of the biosensor shown in FIG. 1.

FIG. 3 is a sectional view for describing a method for measuring a bloodglucose level by using the biosensor shown in FIG. 1.

FIG. 4 is a sectional view for describing a method for measuring a bloodglucose level by using the biosensor shown in FIG. 1.

FIG. 5 is a sectional view of a biosensor according to a secondembodiment of the present invention.

FIG. 6 is a perspective view of the biosensor shown in FIG. 5, fromwhich the cover and the spacer are removed.

FIG. 7 is a sectional view of a biosensor according to a thirdembodiment of the present invention.

FIG. 8 is a sectional view of a biosensor according to a fourthembodiment of the present invention.

FIG. 9 is a perspective view of the biosensor shown in FIG. 8, fromwhich the cover and the spacer are removed.

FIG. 10 is a sectional view of a biosensor according to a fifthembodiment of the present invention.

FIG. 11 is an exploded perspective view showing an example of prior artbiosensor.

FIG. 12 is a sectional view of the biosensor shown in FIG. 11.

FIG. 13 is a sectional view showing the biosensor of FIG. 11 in a statein which blood is introduced into the capillary.

FIG. 14 is a graph showing an example of time course in which theresponsive current suddenly increases when the biosensor shown in FIG.11 is used.

FIG. 15 is a sectional view showing the biosensor of FIG. 11 in a statein which the blood has moved after the introduction into the capillary.

BEST MODE FOR CARRYING OUT THE INVENTION

Biosensors according to a first through a fifth embodiments of thepresent invention will be described below.

First, referring to FIGS. 1 through 4, a biosensor according to thefirst embodiment of the present invention will be described.

The biosensor 1 shown in FIGS. 1 and 2 is a disposable one and mounted,in use, to a concentration measuring apparatus Y, as shown in FIGS. 3and 4.

As shown in FIGS. 1 and 2, the biosensor 1 includes an elongatedrectangular substrate 10, and a cover 12 stacked on the substrate via aspacer 11. In the biosensor 1, the components 10-12 define a capillary13 extending longitudinally of the substrate 10. The capillary 13 isutilized for moving the blood introduced through an introduction port 14in the longitudinal direction of the substrate 10 by capillary actionand for retaining the introduced blood.

The spacer 11 serves to define the height of the capillary 13. Thespacer 11 is formed with a slit 11 a having an open front end. The slit11 a defines the width of the capillary 13. The open front end of theslit 11 a constitutes the introduction port 14 for introducing bloodinto the capillary 13.

The cover 12 is formed with a through-hole 12A. The through-hole 12Aserves to discharge air in the capillary 13 to the outside. The covermay be made of a vinylon, for example, to be entirely hydrophilic.Alternatively, the surface of the cover which faces the capillary 13 maybe hydrophilically treated. The hydrophilization may be performed by theultraviolet radiation or by the application of a surface-active agentsuch as lecithin.

The substrate 10 has an upper surface 10 a formed with a workingelectrode 15, a counter electrode 16, a dummy electrode 17, aninsulating film 18 and a regent portion 19.

The working electrode 15 and the counter electrode 16 are utilized forapplying a voltage to the blood in the capillary 13 or for measuring theamount of electrons supplied from the blood as a responsive current. Theworking electrode 15 and the counter electrode 16 have respective firstends 15 a and 16 a for coming into contact with the blood. The firstends 15 a and 16 a extend widthwise of the substrate 10 and are spacedfrom each other longitudinally of the substrate. The working electrode15 and the counter electrode 16 have respective second ends 15 b and 16b for coming into contact with terminals Ya (See FIGS. 3 and 4) providedin the concentration measuring apparatus Y. The working electrode 15 andthe counter electrode 16 can be formed by screen printing usingconductive paste. Specifically, use may be made of conductive pastecontaining carbon powder, binder resin and a solvent.

The dummy electrode 17 serves to raise the height of the insulating film18 at the most upstream point 12 a of the through-hole 12A of the cover12. The dummy electrode is aligned with the ends 15 a, 16 a of theworking electrode 15 and the counter electrode 16 in the longitudinaldirection of the substrate 10.

The dummy electrode 17 can be formed simultaneously with the workingelectrode 15 and the counter electrode 16 by screen printing, forexample. Therefore, the manufacture of the biosensor 1 does not requirethe additional step for forming the dummy electrode 17, so that adeterioration of the operation efficiency can be prevented.

The insulating film 18 covers most portions of the working electrode 15,the counter electrode 16 and the dummy electrode 17. The insulating film18 includes an opening 18A located in the capillary 13. Through theopening 18A, part of the ends 15 a, 16 a of the working electrode 15 andthe counter electrode 16 and part of the dummy electrode 17 are exposed.The opening 18A has a downstream edge 18 a located almost directly belowthe most upstream point 12 a of the through-hole 12A. Therefore, at thedownstream edge 13 a of the capillary 13, the dummy electrode 17 andpart of the insulating film 18 project upward from the upper surface 10a of the substrate 10 to form a stepped portion 18B. As a result, thesectional area of the capillary at the downstream edge 13 a is madesmaller than other portions. Therefore, at the downstream edge 13 a ofthe capillary 13, blood existing on the upper surface 10 a of thesubstrate 10 is prevented from moving.

The reagent portion 19, which may be in e.g. a solid state, is arrangedto bridge the end 15 a of the working electrode 15 and the end 16 a ofthe counter electrode 16 while closing the opening 18A of the insulatingfilm 18. The reagent portion 19 contains a relatively large amount ofelectron mediator, and a relatively small amount of oxidoreductasedispersed in the electron mediator. As the electron mediator, use may bemade of a complex of iron or Ru, for example. Examples of usable ironcomplex include potassium ferricyanide, whereas examples of usable Rucomplex include one that includes NH₃ as a ligand. The oxidoreductase isselectable depending on the kind of a particular component contained ina sample liquid as the measurement target. For example, the particularcomponent may be glucose or cholesterol. Examples of oxidoreductase forsuch particular components include glucose dehydrogenase, glucoseoxidase, hexokinase, cholesterol dehydrogenase and cholesterol oxidase.

As shown in FIG. 3, the biosensor 1 is mounted, in use, to aconcentration measuring apparatus Y which includes a pair of terminalsYa. Though not illustrated in the figure, in addition to the pairedterminals, the apparatus includes an analytical circuit for analyzingthe blood introduced to the biosensor 1. The terminals Ya are soarranged as to come into contact with the ends 15 b, 16 b of the workingelectrode 15 and the counter electrode 16 when the biosensor 1 ismounted to the concentration measuring apparatus Y. For example, theanalytical circuit has the function of applying a voltage through thepaired terminal Ya to measure the responsive current and the function toperform computation necessary for the analysis of the blood based on theresponsive current.

As shown in FIGS. 3 and 4, when blood BL is introduced into thecapillary 13 with the biosensor 1 mounted to the concentration measuringapparatus Y, the blood BL moves through the capillary 13 by capillaryaction and then stops at the most upstream point 12 a of thethrough-hole 12A of the cover 12.

In the capillary 13, the reagent portion 19 is dissolved by theintroduction of the blood BL, and a liquid phase reaction system isestablished by the electron mediator, the oxidoreductase and the blood,for example. At this time, for example, the particular componentcontained in the blood BL is oxidized, while the electron mediator isreduced. As a result, in the liquid phase reaction system, the reducedproduct of the electron mediator is generated in accordance with theconcentration of the particular component in the blood BL. When avoltage is applied to the liquid phase reaction system through theworking electrode 15 and the counter electrode 16, electrons aretransferred between the reduced product of the electron mediator and theend 15 a of the working electrode 15, for example. In the concentrationmeasuring apparatus Y, the analytical circuit measures the amount of thetransferred electrons as e.g. the oxidation current, and theconcentration of the particular component in the blood BL is computedbased on the measurement result. The computation of the concentration isperformed by applying the measured current to a calibration curveprepared in advance for indicating the relationship between current andconcentration.

In the biosensor 1, the stepped portion 18B provided by the dummyelectrode 17 and the insulating film 18 prevents the blood BL loadedinto the capillary 13 from moving further. Therefore, in the biosensor1, a sudden increase in the current, which is generated by the electrontransfer between the electron mediator and the working electrode 15, isprevented. Therefore, in the biosensor 1, a deterioration of theanalysis accuracy due to the remove of the blood can be prevented.

In the biosensor 1, the bottom of the insulating film 18 is raised bythe dummy electrode 17 at a portion corresponding to the most upstreampoint 12 a of the through-hole 12A of the cover 12. However, the bottomof the insulating film 18 at that portion may be raised by the workingelectrode 15 or the counter electrode 16 by changing the position of theworking electrode 15 or the counter electrode 16.

Next, biosensors according to the second through the fifth embodimentsof the present invention will be described with reference to FIGS. 5through 10. In the figures referred to hereinafter, the elements whichare identical or similar to those of the foregoing biosensor 1 areindicated by the same reference signs, and the description thereof willbe omitted.

FIGS. 5 and 6 show a biosensor 2 according to the second embodiment ofthe present invention and the principal portion thereof.

The biosensor 2 includes a detection electrode 27 in addition to aworking electrode 15 and a counter electrode 16. Instead, the dummyelectrode 17 (See FIGS. 1 and 2) is omitted.

The detection electrode 27, in combination with the working electrode 15and the counter electrode 16, serves to detect whether or not blood BLof the amount necessary for the analysis is loaded into the capillary13. The detection electrode 27 is covered by the insulating film 28except an end 27A. The detection electrode 27 has another end 27B whoseupstream edge 27 b is positioned slightly upstream of the most upstreampoint 12 a of the through-hole 12A of the cover 12.

The insulating film 28 is formed with an opening 28A. The opening 28Aexposes the ends 15 a and 16 a of the working electrode 15 and thecounter electrode 16. The opening 28A includes a downstream edge 28 apositioned directly below the most upstream point 12 a of thethrough-hole 12A of the cover 12.

In the biosensor 2, the downstream edge 28 a of the opening 28A israised by the end 27B of the detection electrode 27, whereby a steppedportion 28B is formed on the substrate 10 at a portion corresponding tothe most upstream point 12 a of the through-hole 12A. Therefore, also inthe biosensor 2, the blood BL is prevented from moving further along theupper surface 10 a of the substrate 10, so that the analysis of theblood BL can be properly performed.

The biosensor may include a pair of detection electrodes, and whether ornot blood BL of the amount necessary for the analysis is loaded into thecapillary may be detected by the paired detection electrodes. In such acase, a stepped portion for preventing the blood from moving further maybe provided by one of the paired detection electrodes.

FIG. 7 shows a biosensor 3 according to the third embodiment of thepresent invention. The biosensor 3 includes an insulating layer 38formed with a projection 38B. The projection 38B is provided at aportion corresponding to the most upstream point 12 a of thethrough-hole 12A of the cover 12.

In the biosensor 3, the projection 38B prevents the blood BL from movingfurther along the upper surface 10 a of the substrate 10, so that theanalysis of the blood BL can be performed properly.

The projection 38B may be provided by either of a conductive member andan insulating member. The projection 38B may be formed directly on theupper surface 10 a of the substrate 10.

FIGS. 8 and 9 show a biosensor 4 according to the fourth embodiment ofthe present invention and the principal portion thereof.

The biosensor 4 includes a substrate 40 formed with a recess 40B. Therecess 40B is circular and has a most upstream point 40 b positioneddirectly below the most upstream point 12 a of the through-hole 12A ofthe cover 12.

Similarly to the first embodiment, the working electrode 15 and thecounter electrode 16 are covered by an insulating film 48 formed with anopening 48A. The opening 48A of the insulating film 48 includes a linearopening portion 481A and a circular opening portion 482A. The linearopening portion 481A extends from adjacent an end edge 40C of thesubstrate 40 to a most upstream point 40 b of the recess 40B. Thecircular opening portion 482A is connected to the linear opening portion481A and has a circular shape for exposing the recess 40B.

In the biosensor 4, the recess 40B provides a stepped portion 48Bdirectly below the most upstream point 12 a of the through-hole 12A ofthe cover 12. In the biosensor 4, therefore, the stepped portion 48B(recess 40B) prevents the blood BL from moving further along the uppersurface 40 b of the substrate 40, so that the analysis of the blood BLcan be performed properly.

The configuration of the recess 40B is not limited to circular. Forexample, the recess may have another configuration such as polygonal.

FIG. 10 shows a biosensor 5 according to the fifth embodiment of thepresent invention.

In the biosensor 5, instead of the recess 40A (See FIGS. 8 and 9) of thebiosensor 4 of the fourth embodiment, a through-hole 50A is formed inthe substrate 50. The through-hole 50A is provided directly below thethrough-hole 12A of the cover 12 and has a configuration correspondingto the trough-hole 12A. Thus, the most upstream point 50 a of thethrough-hole 50A of the substrate 50 is located directly below the mostupstream point 12 a of the through-hole 12A of the cover 12. Thethrough-hole 50A of the substrate 50 can be formed simultaneously withthe through-hole 12A of the cover 12 by punching.

In the biosensor 5 again, by the operation similar to that of thebiosensor 4 (See FIGS. 8 and 9), the through-hole 50A prevents the bloodBL from moving further, so that the analysis of the blood BL can beperformed properly.

In the biosensor 5, the through-hole 50A of the substrate 50 can beutilized for discharging air from the capillary 13. In such a case, thethrough-hole 12A of the cover 12 can be dispensed with.

The through-hole 50A of the substrate 50 is not limited to circular onebut may have another configuration.

Although the above-described biosensors 1-5 are designed for analyzing aparticular component in blood, the present invention is applicable tothe analysis of a particular component in a sample liquid other thanblood, such as urine, saliva or industrial wastewater.

The present invention is not limited to a biosensor for analyzing asample liquid by the electrode method and is applicable to a biosensorfor analyzing a sample liquid by colorimetry.

1. An analytical tool comprising a substrate, a capillary which isformed on the substrate and into which a sample liquid is to be loadedby movement of the sample liquid in the capillary; wherein the substrateis provided with a liquid movement preventer for preventing the sampleliquid loaded into the capillary from moving further.
 2. The analyticaltool according to claim 1, wherein the liquid movement preventerincludes a stepped portion projecting from the substrate.
 3. Theanalytical tool according to claim 2, wherein the stepped portioncomprises a conductive layer formed on the substrate and an insulatinglayer covering the conductive layer.
 4. The analytical tool according toclaim 3, further comprising a plurality of electrodes provided on thesubstrate for applying voltage to the sample liquid.
 5. The analyticaltool according to claim 4, wherein the conductive layer is formed as adummy electrode which does not contribute to the voltage application tothe sample liquid.
 6. The analytical tool according to claim 5, whereinthe dummy electrode is formed simultaneously with the plurality ofelectrodes.
 7. The analytical tool according to claim 4, wherein theplurality of electrodes include a detection electrode for detectingwhether or not the sample liquid of an amount necessary for analysis issupplied into the capillary, and wherein the conductive layer isprovided by the detection electrode.
 8. The analytical tool according toclaim 4, further comprising an air vent for discharging air from thecapillary in moving the sample liquid in the capillary, wherein theinsulating layer includes an opening which exposes part of theelectrodes and which extends along the capillary; and wherein, as viewedin a thickness direction of the substrate, a most downstream point ofthe opening in a flow direction of the sample liquid is located on asame line or almost same line as a most upstream point of the air ventin the flow direction of the sample liquid.
 9. The analytical toolaccording to claim 1, wherein the liquid movement preventer includes arecess provided at the substrate.
 10. The analytical tool according toclaim 9, wherein the recess comprises a through-hole penetrating throughthe substrate.
 11. The analytical tool according to claim 10, furthercomprising an air vent for discharging air from the capillary in movingthe sample liquid in the capillary, wherein the air vent is arrangedcoaxially or generally coaxially with the through-hole in a thicknessdirection of the substrate.
 12. The analytical tool according to claim9, further comprising an air vent for discharging air from the capillaryin moving the sample liquid in the capillary, wherein, as viewed in athickness direction of the substrate, a most upstream point of therecess in a flow direction of the sample liquid is located on a sameline or almost same line as a most upstream point of the air vent in theflow direction of the sample liquid.